First passive recordings from ocean gliders provide insight into whale behavior for some endangered species

Like robots of the deep, autonomous underwater vehicles, or AUVs, are
growing in number and use in the oceans to perform scientific missions
ranging from monitoring climate change to mapping the deep sea floor
and surveying ancient shipwrecks. Another use for these versatile
platforms has now been found: monitoring the lives of whales.

Marine mammals are major predators in the ocean, but little is known
about many of them and how changing ocean conditions affect their
distribution. Traditional ship or aerial surveys rely on human
observers to detect marine mammals, but these observations are limited
to daylight hours and periods of calm seas and good visibility.
As a result, these surveys are time-consuming, inefficient, and
expensive. Marine mammals can also be detected by passively listening
for their vocalizations. Passive acoustic monitoring of marine
mammals is unaffected by weather, but most applications to date have
involved moored or fixed recorders that can assess only when marine
mammals appear in a single location.

Scientists at the Woods Hole Oceanographic Institution (WHOI) have
reported the first use of passive acoustic recorders in an ocean
glider, a type of AUV that looks like a large model airplane and can
work in water depths to 1,000 meters (about 3,300 feet) or more.
The gliders are developed and operated by the WHOI Autonomous Systems
Laboratory, lead by David Fratantoni of the Physical Oceanography
Department.

Gliders move both horizontally and vertically in the ocean and work
around the clock in all weather conditions, typically up to a month or
longer depending on the project. The vehicle carries a variety of
high-resolution sensors to collect oceanographic measurements such as
temperature, salinity, and fluorescence (a measure of phytoplankton
abundance). Gliders also operate silently, which make them ideal for
passive acoustics studies. Researchers say the gliders are a
promising, cost-effective, and efficient alternative to long-term
ship-based studies given ship costs in the thousands of dollars a day.

Mark Baumgartner, an assistant scientist in the WHOI Biology
Department, reported findings from a pilot study involving the
deployment of recorder-equipped gliders today at the biennial Ocean
Sciences Meeting in Honolulu, Hawaii. Baumgartner said vocalizations of
baleen whales, including right, sei, and humpback whales, could be
heard on all of the glider recorders during the five-day project
conducted in 2005 off the coast of Cape Cod, Massachusetts.

The WHOI team custom built digital audio recorders, installed them
inside the gliders, and then deployed the gliders near an aggregation
of right and sei whales. A gale moved through the area just after
the gliders were deployed bringing high winds and 17 foot seas that
made traditional ship-based marine mammal observations
impossible. In contrast, Baumgartner said the gliders operated
“flawlessly”, collecting data continuously through the gale.

Baumgartner and colleagues have found a close correspondence between
changes in the rate of sei whale calls and changes in the near-surface
abundance of copepods, the sei whale's primary prey. The copepods, a
tiny shrimp-like animal, were observed with an echosounder on the
glider. According to the scientists, the changes in abundance
were a result of the copepods’ daily migration from the surface to the
bottom at dawn and from the bottom to the surface at dusk.

“We speculate that sei whales only feed on the copepods at night
when the copepods are near the surface,” he said at a news conference
on new platforms for ocean measurements. “During the day, when the
copepods are near the sea floor, sei whales vocalize more while
socializing or feeding on other prey.”

Baumgartner said the long-term goal is to study where marine mammals go
and what they do over time scales of weeks to months using unmanned
survey vehicles like gliders that are capable of collecting both
acoustic recordings and oceanographic measurements. “Our five-day
pilot project clearly demonstrated that gliders can be used to collect
high quality acoustic measurements. We are now working to extend
the capabilities of the acoustic system to investigate baleen whale
habitat in ways that have not been possible until now.” The
scientists are currently focusing on baleen whales because they are
endangered and at particular risk from fishing gear entanglements and
ship strikes.

Future work will focus on extending the duration of the recordings,
developing a capability to report detections of whale vocalizations to
land-based researchers in real time, and monitoring the high-frequency
vocalizations of other marine mammals, such as dolphins, sperm whales,
and beaked whales.

Funding for this pilot project was provided by the WHOI Ocean Life Institute and the U.S. Office of Naval Research.

Originally published: February 21, 2006

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